Diesel engines typically emit plumes of smoke which inherently contain particulate matter. Measurement of diesel smoke opacity is of importance to a variety of fields including environmentalists, diesel engine manufacturers, service technicians and diesel owners. One method of evaluating diesel smoke uses a beam of light is directed through an exhaust plume, which is partially absorbed, depending upon the density and size of the particles. The amount of light which is blocked while attempting to pass through the plume is a measure of the opacity of the exhaust plume.
Smoke opacity can be used as an indication of proper engine function as well as engine pollution output. The Federal Environmental Protection Agency (EPA) uses diesel smoke opacity measurements during the type approval of engine systems, and some states and municipalities require periodic smoke opacity testing of diesel powered vehicles. Accordingly, it is desirous to have a continuous, real-time measurement system for determining the opacity of a plume of smoke as it exits an exhaust pipe. Furthermore, the smoke opacity is closely related to another measurement of particulate matter within the smoke plume, the smoke density. As some emission test programs require measurement of the smoke density, it becomes important to be able to calculate the smoke density of a plume of smoke in a relatively straight-forward manner. Smoke opacity is related to smoke density according to the following equation as defined by the Beer-Lambert Law: EQU T=e.sup.(-KL)
Where:
T=Transmittance (%), where T=(1-N/100), N=Opacity (%) PA1 e=Base of Natural Logarithm PA1 K=Extinction Coefficient (Smoke Density) PA1 L=Light Path Length
Therefore: ##EQU1##
It can be seen from the above equations that the values necessary to determine the smoke density of a plume of smoke are the transmittance, which is a measure of the amount of light which passes through the plume of smoke, and L, the length of intersection between the exhaust plume and the light beam. Also, additional calculations yield both the particle size and the particle density of the particles contained in the exhaust plume.
Current end of line diesel smoke opacity meters may be divided into two broad categories, each of which has particular limitations. The first such system is a full flow measurement system. The typical full flow system consists of a single beam of light which traverses the smoke plume while passing from a light source to a light detector. The source and detector are located a fixed distance apart, affixed to a rigid hoop or horseshoe bracket which allows the diesel smoke to pass undisturbed between the optical devices. There are several major drawbacks of such a system. First, placement of the device at the center of the smoke plume is critical to accurate test results, yet no means exists to automatically check for accurate placement. Therefore, it is possible that high smoke opacity values will go undetected because of inaccurate placement. For example, if the device is not aligned so that the light beam is directed across a diameter of the smoke plume, the opacity value will be reduced. In such a system, it is difficult to detect whether or not the optical beam is directed across a diameter of the plume. A second drawback of the full flow system is that because the length between the source and detector is fixed while the length across the plume varies in accordance with exhaust pipe diameter, a measured opacity value taken with the full flow system can not be immediately converted into smoke density units. The length of the light path that intersects the smoke must be known in order to determine the smoke density. In order to determine the intersection length, the operator must measure the diameter of the exhaust pipe, then align the source and detector so as to project light beams through that measured diameter.
A second system for measuring the opacity of a plume of smoke is a partial flow measurement system. Exhaust smoke is forced down a probe and hose assembly by engine exhaust pressure or by a separate pump. The smoke then passes through a sampling tube and is swept away by clean air at the ends of the tube. Because the tube is of a fixed length, the opacity measurement of the light passing through the tube is immediately usable for determining the smoke density; however, there remains several disadvantages to such a system. A partial flow measurement system requires a probe, a hose assembly, and possibly a separate pump. This system is more costly, has a slower time response, increases the amount of sample mixing during transport of the smoke through the apparatus, requires much maintenance as particulate matter tends to clog the apparatus, is much larger, is subject to sample condensation when used in cold weather, and is fairly awkward to implement on tall exhaust stacks (e.g., on a diesel truck).